In general, bipolar junction transistors (BJTs) and field-effect transistors (FETs) are constructed using different fabrication steps. When both BJTs and FETs are to be fabricated on the same integrated circuit (IC), the fabrication process is generally referred to as BI-CMOS. (The term BI-CMOS is sometimes specifically restricted to a particular combination of BJTs and FETs, namely, BJTs and CMOS-type FETs. CMOS is an acronym for Complementary Metal Oxide Semiconductor).
In theory, the fabrication of BI-CMOS devices is straightforward. However, in practice, if one merely adds a BJT sequence of steps to an FET sequence, the resulting sequence contains a large number of total steps, many of which are redundant.
In IC fabrication generally, it is desirable to reduce the total number of processing steps.